A study of grid failure mode drivers and methods for accelerated life testing of a 30 cm diameter ion thruster

In view of the high cost caused by the 1:1 lifetime verification test of ion thrusters, the lifetime acceleration test should be considered. This work uses the PIC-MCC (Particle-in-Cell Monte-Carlo Collision) method to analyze the five failure factors that lead to the failure of the accelerator grid of a 30 cm diameter ion thruster under the working mode of 5 kW. Meanwhile, the acceleration stress levels corresponding to different failure factors are obtained. The results show that background pressure has the highest stress level on the grid's erosion. The accelerator grid aperture's mass sputtering rate under the rated vacuum degree (1 × 10⁻⁴ Pa) of 5 kW work mode is 8.78 times that of the baseline vacuum degree (1 × 10⁻⁶ Pa), and the mass sputtering rate under worse vacuum degree (5 × 10⁻³ Pa) is 5.08 times that of 1 × 10⁻⁴ Pa. Under the influence of the other four failure factors, namely, the voltage of the accelerator grid, upstream plasma density, the screen grid voltage and mass utilization efficiency, the mass sputtering rates of the accelerator grid hole are 2.32, 2.67, 1.98 and 2.51 times those of the accelerator grid hole under baseline condition, respectively. The ion sputtering results of two 30 cm diameter ion thrusters (both installed with new grids assembly) after working for 1000 h show that the mass sputtering rate of the accelerator grid hole under vacuum conditions of 5 × 10⁻³ Pa is 4.54 times that under the condition of 1 × 10⁻⁴ Pa, and the comparison error between simulation results and test results of acceleration stress is about 10%. In the subsequent ion thruster lifetime verification, the working vacuum degree can be adjusted according to the acceleration stress level of background pressure, so as to shorten the test time and reduce the test cost.